2752 CAVEN AND SAND: THE DISSOCIATION PRESSURESCCLVI1.-The Dissociation Pressures of the AlkaliBicarbonates. P a ~ t 11. Potassiu~i , Rubidiunz,and Caesium Hydrogen Carbonates.By ROBERT MARTIN CAVEN and HENRY JULIUS SALOMON SAND.IN a former paper (T., 1911, 99, 1359) we recorded the results ofour study of the thermal dissociation of sodium hydrogen carboii-ate, and showed that sodium carbonate monohydrate was notformed from the bicarbonate within the temperature limits of ourexperiments. It was stated, however, by Hermann ( J . pr. Ghem.,1842, 26, 312) that the sesquicarbonate, Na,H,(C03),,3H,0,results when the hydrogen carbonate is heated to 200O. Theformation of the sesquicarbonate from the hydrogen carbonate isdenied by Lescoeur ( A ? m C'him. Phys., 1892, [vi], 25, 423), butwe determined t o establish the simpler manner of decompositionof the hydrogen carbonate under the conditions of our experimentsby the direct estimation of the proportion between water vapourand carbon dioxide in the gaseous phase.For this purpose a glass reservoir of 50 C.C.capacity was joinedon the one side through a tap to the tube leading from the flaskcontaining the heated bicarbonate, and was connected on the otherside with calcium chloride and soda-lime absorption tubes. Thereservoir and absorption tubes were exhausted; the former wasthen heated above looo, and the tap communicating with theAask opened so that the gaseous mixture might expand into thereservoir without aqueous condensation. After a few seconds thetap was closed again, and the water and carbon dioxide now inthe reservoir were carefully transferred to the absorption tubes.It was advantageous to allow some of the water to condense inthe reservoir, and subsequently to remove i t by slow evaporation,and an arrangement was made to carry forward the last traces ofwzter-vapour and carbon dioxide in a stream of purified air.The following results were obtained with sodium hydrogencarbonate :Temperatures Molecularof thermostat.H,O. CO,. ratio H,O : CO,.101.5" 0.0118 0.0295 1 : 1.020.0108 0-0259 1 : 0.98 101.8"These results were sufficient to show that water is not retainedto form the hydrated sesquicarbonate, Na,H,( C03),,3H20, wheOF THE ALKALI BICARRONATES. PART 11. 2753sodium hydrogen carbonate is heated under the conditions of ourexperiments, alt'hough they do not preclude the possibility of com-bination between sodium carbonate and sodium hydrogen carbon-ate to form an anhydrous intermediate salt such as Na,H,(CO&.Po t assiu?n Hydrogen Car b o nat e .It was mentioned in the former paper that Lescceur (*4?717.Chim.I'hys., 1892, [vi], 25, 423 *) measured the dissociationpressure of sodium hydrogen carbonate, obtaining results muchlower than those obtained by us. The apparatus employed byLescoeur (A'nn. Chim. Phys., 1889, [vi], 16, 389) differed in prin-ciple from ours, and consisted of a barometer tube surrounded bya vapour jacket. The substance was contained in a small tube inthe space above the mercury, and gas could be withdrawn fromthis space by means of a narrow open tube which passed down-wards through the mercury.It is noteworthy that the vapourpressuree of potassium hydrogen carbonate observed by Lescceurare slightly higher than ours a t the lower temperatures, butapproach closely to them a t 127O, the highest temperature at whichobservations were made by Le'scceur. These results are shown bythe dotted curve on our diagram.I n the study of the thermal dissociation of potassium hydrogencarbonate the posddity of the formation of a hydrated compoundof normal carbonate and hydrogen carbonate, or of the retentionof water by the normal carbonate, had first t o be considered. Sincethe temperature of sensible dissociation of potassium hydrogencarbonate is considerably higher than that of the sodium salt, theformation or" intermediate compounds containing water seemeda priori less likely than in the case of sodium.The anaIysis of thegaseous phase resulting from the decomposition of potassiumhydrogen carbonate provided, however, direct evidence that waterwas not retained to form such intermediate compounds.Measurements made in the manner described above yielded thefollowing results :Temperature MolecuIarof thermostat. H,O. CO,. ratio H,O : CO,.145" 0.0080 0.0253 1 : 1.28160.2' 0.008'9 0.0232 1 : 1.07161.1" 0.0068 0.0164 1 : 0.98Thus it is shown that potassium hydrogen carbonate dissociates* This reference was wrongly given as Ann. Chim phys., 1893, [vi], 28, 423-into normal carbonate, carbon dioxide, and water2754 CAVEN AND SAND : THE DISSOCIATION PRESSURESThe dissociation pressures of potassium hydrogen carbonate weremeasured in %he wme way as those of sodium hydrogen carbonate,and the results obtained are here tabulated, together with thosecalculated from the equation :logp=a-bjT,where a= 10.832 and b =3420.It should be remarked thaE a t 120° and upwards the vapourpressure of mercury beconies appreciable, and consequently a cor-rection has been applied t o the pressure readings for the highertemperatures.Potassium Hydrogen Carbonate.Pressure in mm.of mercury.Temperature.151.8'156.0147.8137-7127-2119.1104.600.276.363.792.5103.5116-4127.4138.4146.3153.4155.4Rising.610.6733.0--3 1-257.7111.2192.0322.8471.4663.1713.8Falling.-503.1314.7184.1124.156.G24.611.44.1-Calcuiated.60472450632019312059.626.111-04-729.966-0112196330473647706These experimental values are shown in the figure in relation t othe calculated curve, together with those of sodium, rubidium, andcmiurn.It may be pointed out t'hat in this case the experimentalvalues lie along the calculated curve throughout the whole rangeof temperature.Heat of Dissociation of Potassium Hydrogen Carbonate.From the value of the constant b =3420 the heat of dissociationof potassium hydrogen carbonate per gram-molecule of gas pro-duced is calculated by means of the equation q =log, lOBb to be15,730 calories, since log,l0=2-30 and B=2.As in the case ofsodium hydrogen carbonate, the heat of dissociation, x, of twogram-molecules of potassium hydrogen carbonate may be calculatedfrom the thermal equation:2LKHCO-J = [K,CO,] + (H,O 1 + { CO,} - X OF THE ALKALI BICARBONATES. PART 11. 2755The heats of formation from their elements of 1 gram-moleculeof pot'assium hydrogen carbonate and potassium carbonate are,according to de Forcrand (Compt. rend., 1909, 149, 719), respec-tively 231?630 and 275,370 calories, whence, accepting the heats offormation of water as steam a t looo, and of carbon dioxide to be58,060 and 97,000 calories respectively, x= 32,830, instead of31,460, as calculated from our results.Rubidium and Caesium Hydrogen Carbonates.Rubidium and msium ++ hydrogen carbonates were prepared from,the normal carbonates according to de Forcrand's method (Compt.rend., 1909, 149, 719) by exposing concentrated solutions of thelatter salts to an atmosphere of carbon dioxide in a desiccatorcontaining sulphuric acid.I n some earlier experiments on thedissociation pressure of rubidium hydrogen carbonate a small pro-portion of the normal carbonate was mixed with it previous to itsintroduction into the reaction flask, but, owing to the very hygro-scopic nature of the latter salt, it was judged better to produce anamount of it sufficient to secure the satisfactory reversal of thereaction by heating the bicarbonate in the reaction flask itself,and pump?ng out the dissociation products.Since i t had beenshown that potassium as well as sodium hydrogen carbonate yieldsas dissociation products equirnolecular proportions of water-vapourand carbon dioxide, it was at first thought safe to assume t'hatrubidium and cEsium hydrogen carbonates would behave similarlywhen heated; but, owing t o difficulty in interpreting the experi-mental results fo be recorded below, the composition of the gaseousphase was estimated in the case of these salts also.The dekrmination of the dissociation pressures of rubidium andcaesium hydrogen carbonates was carried out in the apparatuspreviously employed. Sixteen grams of the rubidium salt wereheated to atout 160°, and gas was repeatedly withdrawn at thattemperature until successive readings after restoration of pressureagreed. I n the case of caesiuni hydrogen carbonate, about 18 gramswere employed, and the salt was heated to 163O until the pressurebecame constant after successive withdrawals of gas.Retardationeffects, such as were observed in the case of sodium hydrogencarbonate, which necessitated the employment of a much largerquantity of the reacting substance, did not occur with either salt.Owing, however, to the lengthened heating a t high temperatures,and the fact that the temperature of the air above the sulphuric* Cmium carbonate could not be purchased, but fortunately the amount of thissalt available was sufficient for our experiments2756 CAVEN AND SAND: THE DISSOCIATION PRESSURESacid bath was slightly lower than that of the acid in which thethermometer and reaction flask were immersed, distillation ofmercury within the thermometer took place, and consequently thereadings might be several degrees tloo low. This difficulty waspartly overcome by fixing the thermometer so that the top of themercury column was always above the level of the acid, the errorthus introduced being negligible.When, however, small frag-ments of mercury thread appeared in the upper part of the thermo-meter, it was necessary to reject the readings.The following are the experimental results obtained, togetherwith those calculated from the formula log p=u - b /T, where forRbHCO,, n =12712, b =4300, and for CsHCO,, a=16'930,b = 6300.Rubidium Hydrogen Carbonate.Temperature.160'153.5135.3120.5109.397.391.261.512.7106.5120.1137-3146.8151.9161-0170.0164-0158.4151.5143.2135.2121.2112.995.815.011 1.4136-5147-1153.5158.6Pressure in mm.of mercury.Rising. Falling. Calculated.- 594.1 605- 451-7 427 - 197.9 152 - 116.5 60.9 - 75.5 29.2 - 49.3 12.6- 40.9 s.0 - 18.4 0.7 - 0.7 0.051.5 - 24.197.0 - 59.4/ 7211.9 - 171323-1 - 296392 405.9 -623.7 - 6381,038.4 - 1,045 - 750.0 747 - 552.5 556 - 392.2 383 - 247.8 240 - 179.0 151 - 96.3 63.8 - 75.4 37-2 - 49.8 11-3 - 0.0 0.076.0 - 33-6217-1 - 163343-2 - 300446.3 - 427562-4 - 56OF THE ALKALI BICARBONATES. PART 11.Ca esizc m Hydroge,n Car b o m t e .Temperature.163.0"160.1163.1142.7133.611G.5103.8s9-6103.0117.9133.1140.0151.6158.1169.9172.2177.0179.8178.1175.4172.7166.1157-8152.4144.5135.3Tliese results differPressure in mm.of mercury.h .Rising.322.8- ------28.847.579.3100.7168.9234.8502.1599.7847.11,029-5--------Falling.-267.01SO.0115.981.046.525.316.3----------915.1755.0614.1399.8241.0183.3130.691.5in an iinportaiit wayCalculated.30324214059.627.35.71.60.41-56.524.047.41242076076018511,03892 17596243 8220213269.231.62757from those obtainedwith potassium hydrogen carbonate. For whilst the experimentalvalues lie along the curve throughout its whole length in the caseof the potassium salt, it is only the pressures above 158O withrubidium hydrogen carbonate, and above 1 6 5 O with czsium hydro-gen carbonate, that agree with the values calculated from theformulae.Discordant pressure values were obtained a t the lower tempera-tures in the case of sodium hydrogen Carbonate, but these wereshown t o be due t o retardation, ascending values being too low,whilst descending values were too high.Here, however, ascendingand descending values agree; they therefore appear t o indicate atrue cquilibrium. Consequently no single curve of the typelog p=a - b / T can be drawn t o represent the experimental valuesobtained in the dissociation of rubidium and czsiuin hydrogencarbonates, and i t became necessary to investigate the cause of theanomaly.This could best be done by the analysis of the gaseous phase,which was consequeiitly carried out.with both these salts, but ingreater detail with rubidiuiii hydrogen carbonate. About 19 grainsof this latter salt, pulverised and dried over sulphuric acid in 2758 CAVEN AND SAND: THE DISSOCIATION PRESSURESvacuum desiccator for several days, were heated in the reaction flaskto 120° f o r twent,y-four hours; the flask was then exhausted, andany wat'er vapour that had been evolved was drawn off through acalcium chloride tube. The delivery tube was then attached toweighed calcium chloride and soda-lime tubes, and the flask washeated to 170O. The products evolved a t this temperature werepassed through the absorption tubes by maintaining a reducedpressure on the further side until it was judged that sufficientC70, 8Temperatzwc.absorption liad taken place.The flask was then exhausted, andthe whole of the water-vapour and carbon dioxide collected in theabsorption tubes. In two successive experiments 0.0628 and 0-0770gram of water and 0.1400 and 0.1835 gram of carbon dioxide werecollected; these correspond with the molecular ratios H20 : C02=1 : 0.94 and 1 : 0.98. Thus it was shown that water-vapour andcarbon dioxide ar0 evolved in approximately equimolecular pro-portions from rubidium hydrogen carbonate a t 170O.The reaction mixture was then, allowed to cool $ornewhat, aiiOF THE ALKALI BICARBONATES.PART IT. 2759tlie proportions between tlie two components of the gaseous phasewere estimated a t temperatures approaching and correspondingwith those a t which anomalous pressure values appear on the disso-ciation curve. Owing to the small quantities of gas evolved intheso experiments i t was necessary t.0 allow the absorption tocontinue over lengthened periods of time, amounting a t the lowertemperatures to two days ; nevertheless, interesting results wereobtaiiied under parallel conditions, which are liere shown :Temperature. H,O. co,. RIolecular ratio H,O : CO,.145' 0.0197 0.0507 1 : 1.046132 0.0147 0.0343 1 : -957131 0.0053 0.0108 1 : -847127 0.0087 0.0164 1 : -770125 0.0180 0.0336 1 : -763Before a definite interpretation could be given t o the undoubteddeficiency of carbon dioxide or excess of water-vapour in thegaseous phase a t the lower temperatures i t was necessary to knowthe ratio between the amounts of these substances remaining inthe residue.To discover this ratio a fresh experiment was carriedout with 1 gram of carefully dried rubidium hydrogen carbonate,which was heated in the silica flask previously employed in theexperiments with sodium hydrogen carbonate, this flask being usedto avoid the possibility of the absorption of carbon dioxide byglass. I n three successive experiments carried out at 123O tliefollowing molecular ratios were found : H,O : CO, = 1 : 0.731,1 : 0.648, 1 :-0.662; but on ignition of the residue this ratio wasfound to be 1 : 0.995.Thus i t was shown that, excess of waterpresent in the gaseous phase a t the lower temperatures is extrane-ous water, and is not derived from the preferential loss of waterby the salt itself.Similar experiments carried out with a small quantity of c&umhydrogen carbonate pointed to a like conclusion. A t 1 7 3 O and182O the molecular ratios found were H,O : CO,=1 : 1.009 and1 : 1-05 respectively; a t lower temperatures water-vapour wasslowly evolved even after all the carbon dioxide had beenexhausted.It had been concluded that the rubidium, as well as czesiumhydroger, carbonate, prepared for and employed in these experi-ments was pure and dry, because the loss incurred on ignition oftho former saltl closely agreed with theory; but it remained possiblethat this agreement was due t o compensation, and t h a t t'lie productwas really a mixture of rubidium normal and hydrogen carbonates,together with some water.A further experiment in which 1 gramof the salt dried as described above was heated, and all the evolvedproducts were collected and weighed showed that it containe2'760 CAVEN AND SAND: THE DISSOCIATION PRESSURES0.0804 gram of water instead of 0.0614 gram, and 0.1277 gramof carbon dioxide instead of 0.1500 gram, the total loss on heatingbeing 0.2081 gram instead of 0.2114 gram according t o theory.The anomalous pressure values obtained a t the lower temperatureswith rubidium and czsium hydrogen carbonates are probably due,there'fore, to tlhe following cause.The salts employed contained a certain proportion of the normalcarbonates, together with some water tenaciously held in spite ofcareful drying.This water was not present in sufficient amount,however, perceptibly to interfere with the equimolecular pro-portions between the water vapour and carbon dioxide evolvedfrom the bicarbonate a t high temperatures and pressures after theremoval of some of the gas; as was proved by direct experiment(see p. 2758), and by the fact that successive readings agreed.When, however, the temperature was lowered and absorption tookplace, excess of water vapour, although small as regards its abso-lute value, would become relatively great owing to the smallnessof the total amount of gas left, and would thus become operativein greatly increasing the observed pressures.The anomalies that'exist in the lower parts of ths dissociation pressure curves ofrubidium and caesium hydrogen carbonates are therefore ultimatelyto be attributed t o the exceedingly hygroscopic nature of thenormal carbonates of these metals. It may here be pointed out,moreover, that the curves would tend t o become' too shallow onaccount of any small excess of water vapour present in the gaseousphase.Heats of Dissociation of Rqchidz'um and Cesium HydrogenCarbonates.From the equation p=log,lORG, when b=4300 and 6300respectively, the heats of dissociation are calculated to be: forrubidium hydrogen carbonate 19,780 cals. and for czesium hydrogencarbonate 28,980 cals.According t o de Forcrand (Compt.rend., 1909, 149, 719) theheats of formation per gram-molecule are 231,920 and 274,900 cals.for RbHCO, and Rb,CO,, and 232,930 and 274,540 cals. respectivelyfor CsHCO, and Cs+20,; whence the heats of dissociation pertwo gram-molecules of RbHCO, and CsHCO, are calculated to be33,800 and 36,260 cals. respectively, instead of 39,560 and59,960 cals. derived from our results.We are unable to offer any explanation of the discrepancybetween our thermal values and those of de Forcrand. Thesevalues might be approximated by the reduction of the value of bin the equation log p = a- b I;, which would involve a reductioOF THE ALKALI BTCARBONATES. PART 11. 2'761in the steepness of the corresponding dissociation-pressure curves.The increasing steepness from potassium to cmium is well shownin the figure by the increased curvature apparent midway in'thecurves, which is especially apparent with caesium.It has been shown above, however, that the only cause of errorwhich we a t present recognise, that is, the departure from equi-molecular prqportions in the gaseous phase, would have the oppositetendency of making the curves too shallow.Consequently wecannot admit the validity of reducing the values of the constantb in the two equations. On the other hand, whilst we cannotcjffer any general criticism of :he conclusions of de Forcrand, i tmay be pointed out that they are based on diminishing values forthe heats of formation of the normal carbonates from potassiumto czsium, namely, K,CO, = 275,370 cals., Rb,CO, = 274,900 cals.,Cs,CO, = 274,540 cals. ; values which we should certainly a prioriconsider improbable.The results of our experiments on the dissociation pressures ofthe alkali bicarbonates show, not only that the stabilities towardsheat of these salts increase with rise of atomic weight and accom-panying increase in electropositiveness of the metals, but also thatsodium hydrogen carbonate is widely separated from the otherthree salts in stability.Thus there is furnished another example of the fact that abreak occurs in the gradation of properties of compounds of thealkali metals a t the1 point of transition from the short t o the longperiods in the periodic classification of the elements.We desire to express our indebtedness to the Research FundCommittee of the Chemical Society for a grant which has defrayedpart of the cost of this investigation.UNIVERSITY COLLEGE,NOTTIK GHAM.THE SIR JOHN CASSTECHNICAL Iwwrrre, LONDON